Anionic polymerization process

ABSTRACT

A process for anionic polymerization of polymerizable monomers with an organo lithium initiator followed by polymerization termination characterized in that terminated reaction products are neutralized by contacting with sulfuric acid, phosphoric acid or a mixture thereof.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of application Ser. No. 322,443 filed Mar. 13,1989, abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a process for preparing polymers bymeans of anionic polymerization. More particularly the present inventionrelates to a new method for the termination of such anionicpolymerization reactions or for treating coupled reaction products ofsuch polymerization reactions.

It is previously known in the art to prepare polymers of anionicallypolymerizable monomers, especially monovinylidene aromatic monomers andalkadiene monomers via anionic polymerization. Highly useful polymersare prepared in the form of block copolymers containing one or moreblocks of a monovinylidene aromatic polymer and one or more blocks of analkadiene polymer. Suitably such polymers are prepared in an organicsolvent and are usefully employed in adhesive formulations, as modifiersfor thermoplastic resins and asphalt or bituminous compositions or insolvent containing cements or mastic formulations. The polymerscontaining residual unsaturation may be hydrogenated if desired toproduce products having improved weathering resistance.

The initiators used in such anionic polymerizations are typically verystrong bases. Examples are the alkali metal organyl compounds,particularly alkali metal alkyls, especially sodium and lithium alkyls.After polymerization is complete, the terminal monomer group of theliving polymer anion must be terminated. Termination may occur through acoupling reaction by means of a coupling agent or in the event a coupledcompound is not desired by the use of suitable proton donating agentssuch as an organic alcohol, ammonia, amines or even water to terminatethe living anion. However, the remnant formed by this termination isitself a basic species. In the industrial preparation of polymers viaanionic polymerization, especially the preparation of block copolymersof monovinylidene aromatic monomers and alkadienes it is often desirableto include an antioxidant in the polymeric syrup to prevent oxidativeand mechanical degradation of the polymer during devolatilization andfinishing. However, many of the suitable antioxidants employed in suchformulations are sensitive under basic conditions. That is, suchantioxidants react with basic species thereby forming undesirableproducts. Alternatively under basic conditions the antioxidant may beinhibited in its ability to prevent oxidative degradation of theresulting polymer.

Consequently products which have been neutralized by the use of organicalcohols, especially such products further containing an antioxidant,have been found to be lacking in both color retention and in agingstability. Such polymeric products have been found to change in meltviscosity after further mechanical and thermal treatment and to bemarked by an absence of clarity as a result of increased haze. Inaddition such polymeric products tend to have increased yellowing. Also,physical properties, such as ultimate tensile strength, are adverselyaffected.

In U.S. Pat No. 4,415,695 it is proposed to employ boric acid as aterminating agent in an anionic polymerization. Disadvantageously whenboric acid is employed as a terminating agent in the preparation of ablock copolymer of a monovinylidene aromatic monomer and an alkadiene,the resulting product still possesses an undesirable change in meltviscosity upon thermal aging.

It would be desirable if there were provided an improved process forpreparing polymers by means of anionic polymerization techniquesemploying a terminating agent which does not adversely affect thepolymer properties.

It would also be desirable if there were provided an improved techniquefor treating terminated reaction products prepared by anionicpolymerization that allows for the preparation of polymers havingimproved clarity and other polymer properties.

It would be desirable if there were provided an improved technique fortreating coupled reaction products prepared by anionic polymerizationthat allows for the preparation of polymers having improved clarity andother polymer properties.

Finally, it would be desirable if there were provided adhesivescontaining as one component the improved block copolymers ofmonovinylidene aromatic monomers and alkadienes prepared according tothe present invention.

SUMMARY OF THE INVENTION

According to the present invention there is now provided a process forpreparing polymers by means of the anionic polymerization ofpolymerizable monomers the steps of the process comprising:

(a) contacting one or more anionically polymerizable monomers with analkali metal containing anionic initiator under anionic polymerizationconditions;

(b) terminating the polymerization by contacting the reaction mixtureresulting from step (a) with an inorganic acid selected from the groupconsisting of sulfuric acid, phosphoric acid, and mixtures thereof: and

(c) recovering the resulting polymer.

In a further embodiment of the present invention there is provided aprocess for preparing polymers by means of the anionic polymerization ofpolymerizable monomers the steps of the process comprising:

(a) contacting one or more anionically polymerizable monomers with analkali metal containing anionic initiator under anionic polymerizationconditions;

(b) terminating the polymerization by contacting the reaction mixtureresulting from step (a) with a proton donating terminating agent therebyresulting in the formation of an alkali metal salt of the protondonating terminating agent:

(c) neutralizing the alkali metal salt by contacting the reactionmixture resulting from step (b) with an inorganic acid selected from thegroup consisting of sulfuric acid, phosphoric acid, and mixturesthereof; and

(d) recovering the resulting polymer.

In an additional embodiment of the present invention there is provided aprocess for preparing polymers by means of the anionic polymerization ofpolymerizable monomers the steps of the process comprising:

(a) contacting one or more anionically polymerizable monomers with analkali metal containing anionic initiator under anionic polymerizationconditions:

(b) coupling the reaction product of step (a) with a coupling agentthereby resulting in the formation of a coupled polymeric reactionproduct;

(c) contacting the reaction mixture of step (b) with an inorganic acidselected from the group consisting of sulfuric acid, phosphoric acid,and mixtures thereof; and

(d) recovering the resulting polymer.

Also included within the scope of the present invention is a polymericproduct prepared by anionic polymerization according to the foregoingtechniques as well as formulated products such as adhesives preparedtherefrom. In a particular embodiment such a polymerization product alsocontains residual quantities of an alkali metal compound which is theremnant of the polymerization initiator and an amount of an inorganicacid selected from the group consisting of sulfuric acid, phosphoricacid, and mixtures thereof sufficient to neutralize the alkali metalcompound.

DETAILED DESCRIPTION

Processes for the anionic polymerization of monomers are well known inthe art. Initial work utilizing sodium initiators such as sodiumnaphthalene (J. Amer. Chem. Soc., 78, 2656, 1956) was later followed bylithium containing initiators such as secondary butyllithium (U.S. Pat.Nos. 3,321,635 and 3,265,765). More recent process improvements have ledto precise control of the polymerization to produce polymers having awide variety of physical properties. Examples include EP 210,677 whichdiscloses an adiabatic polymerization of styrene in cyclohexane followedby addition of butadiene monomer. Additional anionic techniques includehybrid Ziegler-Natta/anionic methods disclosed in U.S. Pat. No.4,480,075; the use of Lewis bases to control diene vinyl content (U.S.Pat. No. 4,530,985); and various hydrogenation techniques to providesaturated polymeric products (U.S. Pat. Nos. 4,595,749, 4,035,445). Theteachings of all of the foregoing references are incorporated herein intheir entirety by reference thereto.

In addition to monofunctional initiators such as the aforementionedlithium alkyls there have been proposed certain difunctionallithium-containing initiators suitable for the use in the directpreparation of block copolymers of dienes and monovinylidene aromaticmonomers. Such difunctional initiators are disclosed in U.S. Pat. Nos.3,660,536; 3,776,893; 3,954,894; 4,172,190; 4,196,153; 4,200,718;4,205,016; 4,431,777; 4,427,837; and 4,614,768, the teachings of thesereferences are also incorporated by reference thereto. Preferreddifunctional initiators are 1,3-phenylene-bis(3-methyl-1-phenylpentylidene)bis-lithium, the isomeric methylphenyl substitutedderivatives such as1,3-phenylene-bis(3-methyl-1-(2-methylphenyl)pentylidene, 1,3-phenylenebis(3-methyl-1-(4-methylphenyl)pentylidene, etc. and mixtures thereof.

Suitable monomers are those in the class of anionically polymerizablemonomers such as alkadienes, monovinylidenes, alkyl acrylates,acrylamides, acrylonitrile, vinylsilanes, arylsilanes, vinylketones,vinylpyridines, isocyanates, diisocyanates, oxides such as ethyleneoxide, lactams and siloxanes.

Preferred polymerizable monomers for use according to the presentinvention include the well known alkadienes especially butadiene andisoprene, and monovinylidene aromatic monomers, especially styrene andα-methylstyrene as well as ring alkyl substituted derivatives thereof. Apreferred monovinylidene aromatic monomer is styrene.

In the preferred practice of the present invention the phosphoric acid,sulfuric acid or a mixture thereof is combined with the polymer syrupresulting from the polymerization reaction. Such polymer syrup generallycomprises the living anion polymer and a hydrocarbon solvent such astoluene, ethylbenzene, hexane, cyclohexane, or other suitable solventand other additives such as antioxidants, etc. Despite the fact that theorganic phase would generally be predicted to be unaffected by theaddition of an inorganic acid due to partitioning of the respectivecomponents into opposite phases of the mixture it has been surprisinglydiscovered that effective neutralization of the polymer anion resultsaccording to the present invention.

The amount of neutralizing acid added according to the present inventionshould be sufficient to neutralize the polymer anion contained in thereaction mixture. Suitably the equivalent ratio of neutralizing agentadded to the reaction mixture based on initial initiator added (i.e. theratio of equivalents agent/equivalents initiator) is from 0.01:1 to10.0:1, preferably from 0.75:1 to 2.00:1. After about 10.0, preferablyfrom 0.75 to about 2.00. After addition of the neutralizing agent,antioxidant package and additional optional ingredients, the solvent isremoved by devolatilization or other suitable technique and theresulting polymer recovered.

In the further embodiment of the present invention wherein the remnantinitiator is first terminated according to conventional techniquesutilizing a proton donating terminating agent and the resulting basicspecies then neutralized, the addition of acid may take placeconcurrently with the termination process or at a later time. While itmay be possible to obtain thorough incorporation of the phosphoric acidor sulfuric acid if the process is accomplished at the time oftermination, acceptable results are obtained if the polymer product isblended with the neutralizing agent at a later time. Suitably theblending step may be accomplished by use of a ribbon blender, extruder,or other suitable malaxing device. Amounts of neutralizing additiveincorporated into the polymer generally are less than 1.0 percent basedon total polymer weight.

The present invention is also suitably employed in combination with acoupled reaction process. Suitable coupling processes are previouslywell known in the art. A preferred process uses an organic polyhalidecoupling agent such as methylene bromide, 1,2-dibromoethane or silicontetrachloride, etc. After completion of the coupling step the resultingproduct is contacted with the inorganic acid in the previously disclosedmanner. In the case of halide containing coupling agents the process isbelieved to generate the corresponding acid halide and phosphate orsulfate salt which are apparently more compatible with the polymericproduct.

As previously mentioned, antioxidants may be usefully incorporated inthe present products. Suitable antioxidants include those compositionspreviously known and utilized in the art for antioxidant purposes.Examples include hindered phenolic, phosphite, phosphonite, or phosphatetype antioxidants. Other additives such as extrusion aids, UV lightstabilizers, viscosity modifiers, etc. may be incorporated as desired.

Formulated products are readily prepared from the products of thepresent invention as is well known in the art. For example blockcopolymers of monovinylidene aromatic monomers and alkadienes areusefully combined with tackifiers such as hydrogenated terpenes, lowmolecular weight polypiperylidenes, and other suitable compounds to forman adhesive composition.

Having described the invention the following examples are provided asfurther illustrative of the invention and are not to be construed aslimiting.

EXAMPLES 1 AND 2

A five gallon lab reactor is charged with 12.1 kg of cyclohexane, 1.70kg of isoprene and the solution is heated to a temperature of 45° C.Then 19 g of a 0.0282 M/l cyclohexane solution of1,3-phenylene-bis(3-methyl-1-phenylpentylidene)bis-lithium is added tothe reaction solution to remove impurities. Next 380.6 g of the same0.0282 M/l cyclohexane solution of the bis-lithium compound is added toinitiate polymerization. After all the isoprene is polymerized and thereaction mixture has cooled to 60 degrees, 276.4 g of styrene monomerare added to the reaction solution. After all the styrene monomer ispolymerized, 1.84 g of neat isopropanol is added to the reactionsolution. The resulting polymer syrup, containing astyrene-isoprene-styrene triblock copolymer, is then separated intoportions for use in the neutralization and stabilization experiments.Three approximately 1 liter portions of the above polymer syrup areweighed. The resulting polymer syrup is calculated to contain 2.01×10⁻⁶moles Li/g solution.

The amount of neutralizing agent used is calculated by assuming one molebasic species/mole Li. Phosphoric acid is treated as a one proton donor.Sulfuric acid is treated as a two proton donor. The acid concentrationsare obtained from calculations based on reported acid purities,densities, and molecular weights. 0.3 Weight percent of2,2-bis[[3-[3,5-bis(1,1-dimethyl-ethyl)-4-hydroxyphenyl]-1-oxopropoxy]methyl]-1,3-propanediyl3,5-bis(1,1-dimethyl-ethyl)-4-hydroxybenzene propanoate (Irganox® 1010),and 0.3 weight percent of trisnonylphenyl phosphate stabilizer are thenadded.

Devolatilization

The neutralized and stabilized polymer syrups are devolatilized at 90°C. and at 8-25 psi vacuum for 2.5 hr. The polymers are cooled underabout 30 psi vacuum for at least 1 hr.

Melt Viscosity Determinations

Melt viscosity determinations are made using a conventional meltindexer. The melt index determinations are made at 200° C. with a 5 kgweight and a 2.1 mm ×8 mm die (ASTM-D1238, condition 200/5.0). Theresults are shown in Table I.

Ultimate Tensile Measurements

Strips of polymer cut from the devolatilized slab are compression moldedand tensile specimens are die cut. The compression molding conditionsare for 7.0 g sample in a 3"×4.5"×0.035" chase. The press temperature is200° C. Samples are pre-heated for 3.0 minutes under approximately 2000kg/cm² pressure. The samples are then cured for 3 minutes at 18000kg/cm² pressure and then cooled to ambient temperature from 200° C. at arate of 30°-50° C./min. The sample plaques are then die cut using aNAEF® stamping press with an ASTM D 1822, type L die. Ultimate tensilestrengths are measured on a Monsanto T-10 tensometer. Specialelastomeric grips are required to prevent cutting of the sample by theclamping surfaces. The thickness of the sample is recorded to 0.0001"using a micrometer and is inserted into the tensometer so as to attain agauge length of 1.0" (2.54 cm). The samples are pulled at a cross headspeed of 10 in/min (25.4 cm/min) and the tensile value is obtained atbreak. A summary of the data is contained in Table I.

Roll Milling

Portions of the devolatilized polymer slabs are roll milled twice for3.5 minutes at 155° C. at a gap width of 0.028" (0.28 mm).

Tests on Roll Milled Samples

Melt index and in some cases ultimate tensile determinations wereperformed on the roll milled polymers. Melt index and tensiledeterminations followed the same procedure as for the previouslydisclosed slab materials.

Color and Haze Tests

A portion of the plaque made from roll milled polymer was tested foryellowness index and percent haze according to ASTM D 1925-70 and ASTM D1003-61, respectively. A Hunterlab Inc. Tristimulus® colorimeter wasused for both measurements. The instrument was standardized for diffusetransmittance. The data are located in Table I.

                  TABLE I                                                         ______________________________________                                                     Percent Change                                                                After Roll Milling                                                                  Melt     Ultimate                                                                             Yellow                                                                              Trans.                               Example  Neutralizer                                                                             Index a  Tensile b                                                                            Index c                                                                             Haze c                               ______________________________________                                        Comparative                                                                            none      26       32     3.2    28%                                 1        sulfuric  13       15     2.0   22                                            acid                                                                 2        phosphoric                                                                              19.6      6     3.1   23                                            acid                                                                 ______________________________________                                         a = (1 - (Initial MFR/Final MFR)) * 100                                       b = (1 - (Final Tensile/Initial Tensile)) * 100                               c = Roll Milled Polymer                                                  

As may be seen by reference to Table I, the use of sulfuric acid orphosphoric acid to neutralize remnants of the lithium initiator resultedin a product having improved ultimate tensile strength after exposure tofurther processing conditions as well as improved yellowness and hazeafter such processing.

EXAMPLE 3

A five gallon reactor is charged with 12.4 kg of cyclohexane, 566 g ofstyrene, and the solution is heated to a temperature of 45° C. Then 160g of a 0.295 M/l cyclohexane solution of sec-butyl lithium is added toinitiate polymerization. After all the styrene is polymerized and thereaction mixture has cooled to 45° C., 1455 g of Butadiene monomer isadded to the reaction solution. After all the butadiene monomer ispolymerized, 31 g of a 0.0584 M/l cyclohexane solution of dibromoethaneis added over an eight minute period. The resulting polymer syrup,containing a styrene-butadiene-styrene triblock copolymer, is thenseparated into portions for use in the neutralization and stabilizationexperiments. Two portions of approximately 2 liters of the above polymersyrup are weighed. The polymer syrup is calculated to contain 4.19×10⁻⁶moles Li/g of solution.

The two portions are treated as follows. To one sample of polymer syrupno phosphoric acid is added. Phosphoric acid is added to the othersample in a ratio of one mole of acid to one mole of lithium. The acidconcentration is obtained from a calculation based upon the reportedacid purity, density, and molecular weight. 0.5 Weight percent of2,4-bis(n-octylthio)-6-(4-hydroxy-3,5-ditertiarybutylanilino)-1,3,5-triazine (Irganox® 565) and 0.5 weight percent oftris(2,4-ditertiarybutylphenyl) phosphite (Irgafos® 168) are then added.Devolatilization, melt viscosity determination, ultimate tensilemeasurement, and color and haze tests are performed as in Examples 1 and2. The total time of roll milling is 20 minutes for this study. The datafrom these tests are located in Table II.

                  TABLE II                                                        ______________________________________                                                    Percent Change                                                           Acid   After Roll Milling                                                       Ratio    Melt    Ultimate                                                                              Yellow                                                                              Trans.                                Example  to Li    Index a Tensile b                                                                             Index c                                                                             Haze c                                ______________________________________                                        Comparative                                                                            0        20      15      16.3  98.0                                  3        1.0      11      14      7.5   47.1                                  ______________________________________                                         a = (1 - (Initial MFR/Final )) * 100                                          b = (1 - (Final Tensile/Initial Tensile)) * 100                               c = Roll Milled Polymer                                                  

EXAMPLE 4

A five gallon reactor is charged with 12.5 kg of cyclohexane, 285 g ofstyrene, and the solution is heated to a temperature of 45° C. Then 70 gof a 0.295 M/l cyclohexane solution of sec-butyl lithium is added toinitiate polymerization. After all the styrene is polymerized and thereaction mixture has cooled to 45° C., 1565 g of butadiene monomer isadded to the reaction solution. After all the butadiene monomer ispolymerized and the reactor is cooled to 45° C., 285 g of styrene isadded. After all the styrene is polymerized, about 2 ml of isopropanolis added to quench the reaction. The resulting polymer syrup, containinga styrene-butadiene-styrene triblock copolymer, is then separated intoportions for use in the neutralization and stabilization experiments.Two portions of approximately 2 liters of the above polymer syrup areweighed. The polymer syrup is calculated to contain 1.82×10⁻⁶ moles Li/gof solution.

The two portions are treated as follows. To one sample of polymer syrupno phosphoric acid is added. Phosphoric acid is added to the othersample in a ratio of one mole of acid to one mole of lithium. The acidconcentration is obtained from a calculation based upon the reportedacid purity, density, and molecular weight. 0.5 Weight percent ofIrganox® 565 and 0.5 weight percent of Irgafos® 168 are then added.Devolatilization, melt viscosity determination, ultimate tensilemeasurement, and color and haze tests are performed as in Examples 1 and2. The total time of roll milling is 20 minutes for this study. The datafrom these tests are located in Table III.

                  TABLE III                                                       ______________________________________                                                    Percent Change                                                           Acid   After Roll Milling                                                       Ratio    Melt    Ultimate                                                                              Yellow                                                                              Trans.                                Example  to Li    Index a Tensile b                                                                             Index c                                                                             Haze c                                ______________________________________                                        Comparative                                                                            0        39      41      3.1   13.3                                  4        1.0      30      15      3.9   13.4                                  ______________________________________                                         a = (1 - (Initial MFR/Final )) * 100                                          b = (1 - (Final Tensile/Initial Tensile)) * 100                               c = Roll Milled Polymer                                                  

What is claimed is:
 1. An adhesive formulation comprising a polymerprepared by means of a process comprising:(a) contacting one or moreanionically polymerizable monomers selected from the group consisting ofmonovinylidene aromatic monomers and alkadienes with an organolithiumcontaining anionic initiator under anionic polymerization conditions;(b) terminating the polymerization by contacting the reaction mixtureresulting from step (a) with a proton donating terminating agentselected from the group consisting of water and alcohols therebyresulting in the formation of a lithium salt of the proton donatingterminating agent; and (c) neutralizing the alkali metal salt bycontacting the reaction mixture resulting from step (b) with from 0.01to 10.00 equivalents phosphoric acid per equivalent of initiator.
 2. Anadhesive formulation according to claim 1 additionally comprising atackifier.